1 DMA attributes
2 ==============
34This document describes the semantics of the DMA attributes that are
5defined in linux/dma-attrs.h.
67DMA_ATTR_WRITE_BARRIER
8----------------------
910DMA_ATTR_WRITE_BARRIER is a (write) barrier attribute for DMA. DMA
11to a memory region with the DMA_ATTR_WRITE_BARRIER attribute forces
12all pending DMA writes to complete, and thus provides a mechanism to
13strictly order DMA from a device across all intervening busses and
14bridges. This barrier is not specific to a particular type of
15interconnect, it applies to the system as a whole, and so its
16implementation must account for the idiosyncrasies of the system all
17the way from the DMA device to memory.
1819As an example of a situation where DMA_ATTR_WRITE_BARRIER would be
20useful, suppose that a device does a DMA write to indicate that data is
21ready and available in memory. The DMA of the "completion indication"
22could race with data DMA. Mapping the memory used for completion
23indications with DMA_ATTR_WRITE_BARRIER would prevent the race.
2425DMA_ATTR_WEAK_ORDERING
26----------------------
2728DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping
29may be weakly ordered, that is that reads and writes may pass each other.
3031Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
32those that do not will simply ignore the attribute and exhibit default
33behavior.
3435DMA_ATTR_WRITE_COMBINE
36----------------------
3738DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
39buffered to improve performance.
4041Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
42those that do not will simply ignore the attribute and exhibit default
43behavior.
4445DMA_ATTR_NON_CONSISTENT
46-----------------------
4748DMA_ATTR_NON_CONSISTENT lets the platform to choose to return either
49consistent or non-consistent memory as it sees fit. By using this API,
50you are guaranteeing to the platform that you have all the correct and
51necessary sync points for this memory in the driver.
5253DMA_ATTR_NO_KERNEL_MAPPING
54--------------------------
5556DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
57virtual mapping for the allocated buffer. On some architectures creating
58such mapping is non-trivial task and consumes very limited resources
59(like kernel virtual address space or dma consistent address space).
60Buffers allocated with this attribute can be only passed to user space
61by calling dma_mmap_attrs(). By using this API, you are guaranteeing
62that you won't dereference the pointer returned by dma_alloc_attr(). You
63can treat it as a cookie that must be passed to dma_mmap_attrs() and
64dma_free_attrs(). Make sure that both of these also get this attribute
65set on each call.
6667Since it is optional for platforms to implement
68DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
69attribute and exhibit default behavior.
7071DMA_ATTR_SKIP_CPU_SYNC
72----------------------
7374By default dma_map_{single,page,sg} functions family transfer a given
75buffer from CPU domain to device domain. Some advanced use cases might
76require sharing a buffer between more than one device. This requires
77having a mapping created separately for each device and is usually
78performed by calling dma_map_{single,page,sg} function more than once
79for the given buffer with device pointer to each device taking part in
80the buffer sharing. The first call transfers a buffer from 'CPU' domain
81to 'device' domain, what synchronizes CPU caches for the given region
82(usually it means that the cache has been flushed or invalidated
83depending on the dma direction). However, next calls to
84dma_map_{single,page,sg}() for other devices will perform exactly the
85same synchronization operation on the CPU cache. CPU cache synchronization
86might be a time consuming operation, especially if the buffers are
87large, so it is highly recommended to avoid it if possible.
88DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
89the CPU cache for the given buffer assuming that it has been already
90transferred to 'device' domain. This attribute can be also used for
91dma_unmap_{single,page,sg} functions family to force buffer to stay in
92device domain after releasing a mapping for it. Use this attribute with
93care!
9495DMA_ATTR_FORCE_CONTIGUOUS
96-------------------------
9798By default DMA-mapping subsystem is allowed to assemble the buffer
99allocated by dma_alloc_attrs() function from individual pages if it can
100be mapped as contiguous chunk into device dma address space. By
101specifying this attribute the allocated buffer is forced to be contiguous
102also in physical memory.